Liver is the major site of metabolism, and there are a multitude of known human genetic disorders due to hepatic deficiencies. These include phenylketonuria, glycogen storage diseases, urea cycle enzyme deficiencies, hemophilia, alpha-1-antitrypsin deficiency, etc. In these well characterized monogenic diseases, the metabolic imbalance can be restored by proper expression of the missing functions in the hepatocytes of the patients, thereby effecting a permanent cure of the disorders by hepatic gene therapy. As the human genes responsible for many of these disorders are being isolated and shown to be functional after transfection into mammalian cells, the limiting step is undoubtably the lack of suitable technologies to reintroduce these genes into the patient's liver cells. During the past several years, we and others have reported successful isolation of primary hepatocytes and demonstrated that functional genes can be efficiently transduced into them by retroviral mediated gene therapy. Most recently the feat of reintroducing these cells back into experimental animals and demonstrating long-term survival and functional of the engrafted cells in vivo has been accomplished. Primary mouse hepatocytes were effectively transplanted into recipient mice through direct injection into the portal vein or the spleen, and we demonstrated unambiguously that the engrafted hepatocytes migrate to the liver, incorporate themselves into the normal parenchyma and continue to function indefinitely in vivo. Based on this break through, we propose to 1) attempt to correct certain hepatic deficiencies in two different animal models, 2) investigate the regulation of hepatocyte growth and differentiation, and 3) explore and perfect hepatocyte transplantation technologies for larger animals. This ambitious program pulls together a group of investigators with expertise in molecular biology and genetics, cell biology and differentiation, as well as surgery and pathology, who will function as a unit to expedite the development of necessary technologies and clinical protocols for the correction of hepatic deficiencies by gene therapy.